The present invention relates generally to flat-panel displays, and more particularly to large, high-resolution displays of the type in which data are input to picture-elements by means of electrical signals applied to the ends of data buses extending through a seal.
Transmission-line losses in a data bus of a flat-panel display result in attenuation of input signals on the bus. The losses also create picture-quality degradation, which typically takes the form of spatial nonuniformity.
These losses are a function of the capacitance and resistance of the data bus. The metal forming the line should be of low resistance especially for large size displays, which have long buses, and for high spatial resolution displays, which have narrow buses. Suitable metals having the appropriate conductivity include aluminum, copper, gold and silver.
In general, copper is considered to be unsuitable for use in displays based on emission of photons from cathode luminescent phosphors. This is because copper tends to cause uncontrolled color shifts as a contaminant in some phosphors. Gold and silver are expensive materials to be used extensively in cost-sensitive applications such as consumer display products.
Aluminum is a common, high-conductivity metal, widely used in low-cost consumer semiconductor and liquid-crystal display applications. However, thin film aluminum is subject to a condition known as Hillock formation, which is grain-growth in a direction orthogonal to the plane of the film. These Hillocks, if allowed to grow, can cause inter-electrode shorts.
One prior art technique to reduce Hillocks in the manufacture of an Active-Matrix Liquid-Crystal Display (a "AMLCD") is to use an alloy additive to the aluminum thin film. For large size and high resolution AMLCDs, the high-conductivity bus metal of choice is generally aluminum. When aluminum is used as the buried gate electrode in AMLCD (inverted-gate) Thin Film Transistor (TFT), Hillocks can cause inter-electrode shorts or defective transistors. Kawamura in the 1997 Proceedings of the Japan Display Conference reported the use of aluminum alloyed with zirconium to minimize Hillock formation at the expense of nearly doubling the resistivity. Aluminum is sometimes cladded with other metals to suppress Hillocks. Higachi in the 1996 SID Digest reported the use of aluminum cladded with molybdenum/tantalum, again increasing the resistivity of the metal. Thus, although alloying reduces the incidence of Hillocks, it introduces the unwanted effect of increasing bus resistivity.
A Field-Emission Display (a "FED") is typically characterized by a matrix of electron emitters, enclosed in a high-vacuum cavity bounded by an emitter substrate and a viewing screen. The cavity is sealed at its perimeter.
A constraint unique to FED and other high vacuum displays is the requirement of data buses to provide electrical continuity from the active area of the display, which is the area under vacuum, to the region of the display where electrical connection is made to the driving circuitry. Particularly in FED's, the requirement for vacuum in the range of 10.sup.-7 Torr dictates a high-temperature exhaust/sealing process. The seal material is typically a glassy material whose melting temperature is substantially lower than the temperature at which thermal damage would occur to any components of the device, which includes the aluminum buses. However, this sealing glass, or frit, is a good solvent for many materials, including aluminum, at its working or melting temperature. The dissolution of the metal bus in the frit-seal region can cause an unacceptable increase in bus resistance.
Note that the AMLCDs have problems less severe than the FEDs. In AMLCDs, the sealing processes are generally executed using low-temperature epoxy type sealing materials. These sealing processes have little effect on pure or alloyed aluminum, but are unsuitable for high-vacuum applications. Also, unlike the FEDs, other displays based on matrix-addressed device technologies, such as plasma and vacuum fluorescence ones, typically are in a size/resolution domain where one can use lower conductivity materials for the buses. This, in turn, allows the use of more robust materials to extend through the frit seal.
A number of materials, such as thin-film gold, platinum and tungsten, are insoluble in the frit. However, they are not "wetted" by the frit, and thus make a poor vacuum seal.
Another requirement for the data lines is that external electrical signals should be applied to the data bus pads without appreciable contact losses.
It should be obvious that for large-size, high-resolution FEDs, it is necessary to create low-resistance data buses with good frit-sealing and contact bonding properties. Additionally, if aluminum is selected to be the metal for the buses, Hillock formation should be significantly reduced.